In recent years, electrochemical syntheses have been attracting increased attention from the chemical synthesis industry. Many starting materials behave in a unique manner when placed in an electrochemical environment, and hence there are valuable organic and inorganic compounds which can be produced in a relatively simpler and more convenient manner by electrochemical means as compared to conventional chemical means. For example, the conventional chemical synthesis of aldehydes and ketones from alcohols may require substantially anhydrous starting materials and elevated temperature or pressure conditions, whereas an electricity-producing electrochemical (i.e. "electrogenerative") conversion of ethanol to acetaldehyde can be carried out at very modest temperatures (e.g. 25.degree.-90.degree. C.) and pressures (e.g. atmospheric pressure) and with substantial amounts of water present in the alcohol feed material. See, for example, Langer et al, Pure & Appl. Chem. 58, 895, 902-905 (1986).
As will be apparent from the patent and scientific literature relating to electrochemical syntheses, there are at least three modes in which these syntheses can be carried out. The first mode could be called "electrolytic" and is by far the most commonly used. Typical examples of electrolytic syntheses include the manufacture of chlorine and sodium hydroxide from brine, the manufacture of adiponitrile or ethylene glycol by reductive dimerization, and all of the myriad forms of electroplating. Favorable thermodynamics are not required for this mode of electrosynthesis.
A second mode is referred to in the scientific literature as "electrogenerative" or "galvanic" and is characterized by the production of "byproduct" electricity along with the desired chemical product. Favorable thermodynamics (.DELTA.G&lt;O) are essential here, but the energy-producing aspect of this mode is desirable, e.g. in plant siting, since a nearby cheap source of electrical power is not always available.
There can be considered to be even a third mode which is intermediate between "electrolytic" and "electrogenerative" . In this third mode (referred to in the literature as "voltameiotic"), the power requirements of an electrolytic process are substantially reduced but not necessarily eliminated. For example, it has been shown that the energy requirements for the electrolytic production of hydrogen from water can be significantly reduced if a carbon-containing substance is oxidized at the anode while protons are reduced to hydrogen at the cathode.
In any of these electrochemical synthesis modes, an electrocatalyst may be required for efficient operation of the electrochemical cell, i.e. the electrochemical synthesis reactor. Indeed, in the case of reduction of gaseous oxides or oxidation or reduction of organic starting materials, an electrocatalytic surface at the cathode and/or the anode may be essential for a commercially viable process. The electrocatalytic surface, of course, improves the kinetics of the electrochemical system, but improved reaction rates are not always entirely beneficial. Particularly in the case of synthesis carried out in the electrogenerative mode, it may be very difficult to control the composition of the product effluent and/or the composition of products formed in the cell electrolyte. If the sole objective were the recovery of electric power, it would be desirable for the oxidation of organic materials to proceed as far as possible, preferably all the way to carbon dioxide. (This is exactly the principle upon which organically-fueled fuel cells are based.) Similarly, the reduction of nitrogen oxides should proceed all the way to ammonia, if possible, when maximum electrical energy production is desired. However, the vast development of the chemical synthesis industry throughout much of this century has been dependent upon finding catalysts which permit the recovery of products from an intermediate stage of a thermodynamically-favored reaction. (In the "electrogenerative" mode, the oxidation or reduction is by definition thermodynamically favored.) Accordingly, the electrochemical synthesis industry also is in frequent need of electrocatalysts which accelerate reactions without causing them to proceed without selectivity. Conversely, it is important to develop electrocatalysts which are not so completely "fouled" by reactants and/or intermediate or final products as to become inactive and useless, thus necessitating a frequent and usually expensive catalyst recycling operation.
In very recent years, it has been discovered that the phenomena associated with catalyst "poisoning" can actually have beneficial aspects. Most metal catalysts are transition elements which have strong affinities for oxygen, sulfur, and nitrogen compounds, carbonyl groups, and the like. Sulfur-containing impurities in starting materials are a frequent cause of catalyst poisoning, e.g. in the petrochemical synthesis industry. Indeed, the poisoning of metal catalysts by sulfur or sulfur compounds is a serious problem in many chemical processes, and complete poisoning of a catalyst such as platinum black or supported platinum may bring the synthesis operation to a virtual halt. On the other hand, it is now known that partial poisoning of a metal catalyst will produce changes in catalytic activity without rendering the catalyst useless. In the case of conventional metal catalysts such as particulate platinum having a particle size from 500 to 1130 nanometers (on alumina), the "faceting" effects brought about by partial sulfur poisoning may be beneficial. Se, for example, P. J. F. Harris, Nature, 323, 792-794 (October 1986). Harris was able to achieve partial poisoning of the platinum/alumina catalyst film by placing the film in contact with hydrogen sulfide in hydrogen and heating to 500.degree. C. at which temperature the hydrogen sulfide presumably decomposes to form sulfur in situ on the catalyst surface.
T. E. Fischer et al, J. Catal. 53, 24 (1978) were among the first to investigate the effects of adsorbed sulfur on NO and CO adsorption and NO+CO reaction on Pt (100) single crystals. They found that a C(2.times.2) saturation sulfur coverage blocked NO adsorption. Lower coverages permitted NO adsorption on the free surface sites while inhibiting NO dissociation. Interpretation of these results involved both steric and electronic effects. Y. Matsumoto et al. J. C. S. Faraday I, 76, 116 (1980) studied the effect of adsorbed sulfur on N adsorption and dissociation on a polycrystalline Pd foil. In agreement with T. E. Fischer et al, they observed facile NO adsorption on free sites at submonolayer sulfur coverages, with NO dissociation occurring only at low sulfur coverages (.theta.&lt;0.3).
Investigations of similar phenomena in the electrocatalyst field are surprisingly few, and very little is known regarding beneficial effects upon electrocatalysts obtained through a partial poisoning technique. It has been discovered by Pate et al and Langer et al that the presence of sulfur oxides does not necessarily prevent effective reduction of nitric oxide at the cathode of a hydrogen/nitric oxide electrogenerative reactor. See, for example S. H. Langer et al, Ind. Eng. Chem Process Des. Dev., 22, 264 (1983); K. T. Pate et al Environ. Sci. Technol. 19, 371 (1985). These investigations suggest that the presence of SO.sub.2 or CO in the reactor feed (Pt-black cathode) altered selectivity to favor hydroxylamine production rather than ammonia.
Given the present state of the electrocatalyst art, however, the guidelines for implementing a sulfur treatment of an electrocatalyst and for utilizing the resulting electrocatalyst material in an electrochemical synthesis catalyst are unquestionably insufficient for the development of a treatment which will provide beneficial effects rather than simple poisoning or inactivation of the electrocatalyst surface.